In recent years, intensification of environmental pollution control measure against exhaust gas exhausted from an internal combustion engine has been required. Since enhancement of ignitability (ignition performance) contributes to purification of the exhaust gas, a spark plug whose ground electrode is provided with, on an inner surface thereof, an electrode chip (a discharge portion) that is formed using noble metal having high resistance to spark erosion, so as to protrude toward a center electrode, has been developed. In the spark plug having such configuration, in comparison with conventional spark plugs, since the ground electrode can be set away from a spark discharge gap, a flame nucleus (a flame core) formed in the spark discharge gap is less prone to reach the ground electrode at an early stage of its growth process. For this reason, a so-called quenching action, which inhibits the growth of the flame core by that fact that the flame core reaches the ground electrode and heat is absorbed by the ground electrode, is reduced, and thereby improving the ignition performance of the spark plug.
In such spark plug, because a great thermal load is applied to the electrode chip, there is a risk that a crack or separation will appear at a junction portion between the discharge portion and the ground electrode. Thus, for the junction between the discharge portion (an ignition portion) and the ground electrode, a pedestal portion (a projection), as an intermediate member that has an intermediate coefficient of linear expansion between both linear expansion coefficients of the discharge portion and the ground electrode, intervenes between the discharge portion and the ground electrode. With this pedestal portion, thermal stress that could occur in each junction portion of the discharge portion, the pedestal portion and the ground electrode is relaxed, thereby reducing the occurrence of the crack or the separation (for example, see Patent Document 1). Further, in the Patent Document 1, a junction between the electrode chip and the intermediate member is welded not by resistance welding by which an excessive pressure welding force acts on the junction upon the welding but by laser welding which can easily concentrate heat onto the junction and set a melting depth to be deep also reduces a tendency for internal stress to remain after the welding. Then by this laser welding for welding the electrode chip and the intermediate member, a melting portion in which their respective constituent materials (components) are mixed together is formed between the electrode chip and the intermediate member.